The difference is microstructure and alloy content. A stainless steel is any steel with at least 11.5% chromium, which is what gives it resistance to general corrosion. Super duplex stainless steel is a specific high-alloy type, based on around 25% chromium, with a balanced austenitic-ferritic microstructure.
Stainless steels fall into several families, categorised by their microstructure and properties: austenitic, ferritic and martensitic (precipitation hardened). The microstructure largely defines the physical properties.
Austenitic stainless steels
Austenitic stainless steels generally provide excellent corrosion resistance, good mechanical properties across a wide temperature range including low-temperature toughness, and are non-magnetic. Ferritic stainless steels are generally lower in corrosion resistance and cost but resist stress corrosion cracking. Martensitic stainless steels are generally very high strength.
Duplex and super duplex stainless steels
Duplex stainless steels balance austenitic and ferritic microstructures for a favourable mix of mechanical and physical properties. The first duplex grades were developed around 22% chromium. Raising the chromium level to 25% increases strength and corrosion resistance; grades at this level are called super duplex stainless steels. Pushing chromium as high as 32% produces hyper duplex stainless steels.
We carry an extensive range of duplex, super duplex and high-performance austenitic stainless steels.